Motor Control
Starting
Disconnect Means, Controller, Starting Method
Running
Motor Protection, Wire Protection, Speed Control
Stopping
Coasting, Braking
Motor Starters
The starting mechanism that energizes the circuit to the motor There are varying devices that can be used but only two starting methods for motors.
Across the Line Reduced Voltage
Across the Line Start
Connect motor windings to line voltage.
Simplest & lowest cost way to start motor High starting torque and high starting current.
Two Types
Manual Motor Starter Magnetic Motor Starter
Reduced Voltage Starting
Apply below line voltage to motor windings during starting period then connect to line voltage once started and up to speed.
Reduced starting current & reduced starting torque. Reduce Mechanical Starting Shock
More equipment and higher cost
Reduce Inrush Current
Increase Power Quality
Reduces the in-rush current an voltage flicker on the electrical system when starting the motor. (good) Reduces the motors starting torque. (not so good)
Consideration for shutting down the load/system must be made to ensure it can be started the next time.
Reduce Mechanical Starting Shock
Applying full starting torque immediately tends to jerk the system to speed quickly.
Breakable items can be jostled. Drivelines and parts can be damaged.
Reduced torque applied to the system at starting results in a smoother soft start.
Conveyor belts Complicated drive trains with mechanical connections.
Motor Protection
Overcurrent Protection
Protect motor & circuit from very high short circuit currents.
Overload Protection
Protect motor & circuit from overload currents.
Other
Low/high voltage, phase failure, phase reversal, ground fault, bearing temperature monitors, winding temperature monitors, surge suppression, etc.
Overcurrent Protection
Must size a fuse/breaker large enough to start the motor but protect from dangerous overcurrents caused by shorts and ground faults. Generally results in too large a device to protect from overloaded conditions.
Sizing Motor Protection
Find the motors Design FLA in the NEC
1 Phase: Table 430-148 3 Phase: Table 430-150
NEC Requirements, Table 430-152
Standard Fuse Time-delay Fuse Instantaneous Trip Breaker Inverse Time Breaker 300% of FLA 175% of FLA 800% of FLA 250% of FLA
Standard Fuse
Sizing to 300% of NEC FLAthen the next highest standard size. Exception allows the next size larger when the motor will not start. Will hold 500% of rating for approximately second.
Time Delay Fuse
Size to 175% of NEC FLA, select next standard higher size. Hold 500% of their rating for up to 10 seconds. Can be sized much lower than NEC requires and provide backup overload protection.
Inverse Time Breaker
Have both thermal and magnetic features set to strip at standardized levels. Most common breaker used in the industry. Time to trip varies with amp & volt rating of breaker. Will start any motor with Locked Rotor Current below 300% of its rating.
Instantaneous Trip Breaker
Used where timedelay fuses or standard circuit breakers will not hold the starting current of the motor. NEC allows sizing up to 800% of FLA. Some have adjustable settings.
Overload Protection
Protects the motor circuit & motor from overloading conditions when the motor is running. The larger the overload, the more quickly the temperature will increase to a point of damaging the insulation on the motor windings. NEMA: motor survive a 150% overload for 2 minutes when motor is at normal operating temperature
Motor Heating Curve
Full Load Amps (%)
600 500 400 300 200 100 0
Motor Damage Area
Allowable Operation Area
0 1 2 3 4 5 6 7 8 9 10 11 12
Time (Minutes)
Overload Types
Bimetallic & Melting Alloy Overloads Heaters & or Heat cause a contactor to open Solid State Overloads Respond to current going to motor, preprogrammed for certain characteristics similar to standard overloads Electronic Overloads Microprocessor Driven monitoring of current on all phases to motor. Wide range of adjustment and calibration as well as communication
Heater Trip Characteristi
1000
Trip Time (Seconds)
100
10
1 0 200 400 600 800 1000 1200
Rated Current (%)
Bimetallic & Melting Alloy
Advantages
Responds to total heat it sees (combination of ambient & temperature rise) Simple, known technology Low cost Simple troubleshooting
Disadvantages
Located at starter and not motor (may be different temperatures Trip Variability (slow/accuracy) No additional protection other than overloads (low/high voltage, single phasing, phase unbalance, etc.)
Solid State Overloads
Advantages Ambient insensitive Wide FLA adjustment to individual motors No heaters Wide temperature range Disadvantages Set points are still extreme Generically protects while trying to prevent nuisance tripping. Many dont protect against single phasing or unbalance Cost
Electronic Overloads
Advantages
Set trip levels, trip classes and time delays Protection from all types of voltage variations including low/high, ground fault, single phasing, phase reversal and phase unbalance. Communications with controllers & PLCs
Disadvantages
Complexity Cost
Sizing Overload Devices
NEC rules allow two methods:
Calculation
Based on Motors FLA
Temperature Rating 40 Deg C 50 Deg C Will Not Start?
Service Factor up to 1.15 > 1.15 115% 115% 140% 125% 125% 140%
Mnfg. Chart in Starter Cover
Example
FLA = 22, S.F. = 1.00, AMB = 40 Deg C Whats the minimum overcurrent protection device size? Whats the minimum overload protection device size?
Step 1. NEC Full Load Amps
Find NEC Design Amps: NEC Table 430-150
3 Phase, 10 Hp, 230 Volt Amps = 28 amps
Step 2. Conductor Size
Whats the minimum conductor size? NEC 430-22: Ampacity = 125% of FLA
125% X 28 amps = 35 amps
Need a conductor with an ampacity of 35 amps.
From NEC 310-16: #8 AWG Copper
NEC does this so #8 AWG copper will be large enough for any 10 Hp motor in the future.
Step 3. Overcurrent Device
NEC Table 430-152: Standard Fuse
28 amps X 300% = 84 amps, 90 amps 50 amps 225 amps 80 amps
Time Delay Fuse
28 amps X 175% = 49 amps,
Instantaneous Breaker
28 amps X 800% = 224 amps,
Inverse Time Breaker
28 amps X 250% = 70 amps,
Step 4. Overload Device
Use the Nameplate FLA: For S.F. = 1.0, AMB = 40 deg C
22 amps X 1.15 = 25.3 amps
If this does not allow the motor to start, increase to a maximum of:
22 amps X 140% = 30.8 amps
Ambient Compensated Overloads
Standard overload devices are designed for maximum ambient temp of 104 deg F. Above: Nuisance Tripping Below: No Protection Ambient Compensated Devices have a flatter temperature response and should be used in outdoor locations.
Ambient Temperature Correction
140
Rated Current (%)
130 120 110 100 90 80 70 60 20
Non-Compensated Compensated
Standard Rating, 40 C
40
60
80
100
120
140
160
Ambient Temperature (F)
Other Protection Devices
Low/High Voltage Protection Phase Failure Protection Phase Reversal Protection Ground Fault Protection Monitors
Bearing Temperature Monitors Winding Temperature Monitors Vibration Monitors
